1. Field of the Invention
This invention relates to a method for hydrogenating polymers containing ethylenic unsaturation. More particularly, this invention relates to a process for selectively hydrogenating ethylenic unsaturation contained in a polymer comprising ethylenic unsaturation and one or more cyano groups.
2. Prior Art
Polymers containing both ethylenic unsaturation and cyano groups are, of course, well known in the prior art. Most frequently, such polymers are prepared by copolymerizing a polyolefin, particularly a diolefin, and an ethylenically unsaturated nitrile such as acrylonitrile. Such polymers may also incorporate other monomer units such as monomers which would incorporate aromatic unsaturation into the polymer. Polymers prepared by copolymerizing a conjugated diolefin, particularly butadiene, with an ethylenically unsaturated nitrile, particularly acrylonitrile are, of course, well known in the prior art and such polymers have been produced commercially for some time. Perhaps the most common use for such polymers is as a synthetic rubber but such polymers are produced in a rather broad range of relative compositions over a broad range of molecular weights and have a wide range of end use applications. For example, such polymers are known to be useful in fibers, packings, seals, tubes, membranes, sheaths and the like. As is well known in the prior art, however, the polymer generally cannot be used neat in many of these end use applications due to poor oxygen and ozone resistance and to poor thermal stability.
It is, of course, known in the prior art to improve oxygen and ozone resistance as well as thermal stability by selectively hydrogenating the polymer thereby reducing the amount of ethylenic unsaturation contained therein. Early, heterogeneous catalyst comprising a supported metal, such as taught in United Kingdom Pat. Nos. 2,011,911 and 2,087,403, were used. Various metals or metal compounds, which may be used supported or unsupported, are also taught in U.S. Pat. No. 3,700,637. These catalysts are not, however, particularly active and many are not particularly selective and, as a result, high degrees of ethylenic unsaturation conversion frequently are not achievable without converting at least a portion of the cyano groups to amine groups. Later, mixed metal catalysts, particularly palladium in combination with one or more other metals, such as taught in U.S. Pat. No. 4,337,329, have been used but these catalysts too are not particularly active and, again, many are not particularly selective. More recently, it has been proposed to use a metal such as platinum, palladium, ruthenium and the like supported on silica as taught in U.S. Pat. No. 4,452,951 but these catalysts also frequently result in conversion of at least a portion of the cyano groups when a high degree of hydrogenation is achieved and the activity, particularly, when Pd is used remains relatively low. Still more recently, it has been proposed to use a catalyst prepared by combining a palladium carboxylate and a reducing agent such as hydrogen, as taught in U.S. Pat. No. 4,510,293, but still conversion of the cyano groups to amine groups frequently occurs at high conversion. Moreover, this catalyst, particularly, as prepared with hydrogen as the reducing agent is not particularly active. Even more recently, it has been proposed to use various ruthenium compounds as taught in U.S. Pat. No. 4,631,315 and the use of various rhodium complexes is known as taught in United Kingdom Pat. No. 1,558,491 and U.S. Pat. Nos. 3,700,637 and 4,647,627. At least certain of these latter catalysts do facilitate particularly selective hydrogenation of such polymers but the catalyst metals are either rather expensive or in short supply. In light of these deficiencies, then, the need for an improved catalyst composition which can be used to selectively hydrogenate polymers containing ethylenic unsaturation and one or more cyano groups which will permit relatively high conversion (saturation) of the ethylenic unsaturation in relatively short periods of time with reduced conversion of the cyano groups is believed to be readily apparent.